The present invention relates to a gasoline direct-injection engine.
Conventionally, gasoline direct-injection engines which supply a fuel at least containing gasoline into a cylinder and cause the fuel to self-ignite by compressing mixture gas have been known.
For example, JP2013-057268A discloses an engine for causing a self-ignition combustion of a fuel. In order to reduce a pressure increase rate within a cylinder (a rate of a pressure change inside a cylinder with respect to a crank angle change), the engine controls a combustion timing so that a timing at which the pressure increase rate during motoring of the engine becomes the largest negative value (lowest rate) overlaps with a combustion period. Specifically, the pressure increase rate during the motoring becomes its highest value slightly before a compression top dead center (CTDC), becomes zero at the CTDC, becomes a negative value after the CTDC, and then eventually becomes the largest negative value. In other words, the engine retards an ignition timing of the self-ignition combustion to a predetermined timing on the expansion stroke to overlap the combustion period with the timing at which the pressure increase rate during the motoring becomes the largest negative value. Thus, the pressure increase rate in the combustion is reduced and a vibration noise (NVH) level is reduced.
Note that the phrase “during motoring of the engine” used herein indicates a state of the engine with no combustion while a crankshaft of the engine is rotated by an electric motor, and it is used in research studies for, for example, a comparison of the pressure increase rate within the cylinder based on whether or not there is combustion.
However, since an in-cylinder temperature decreases as the expansion stroke progresses, if the ignition timing is retarded excessively, there is a risk of causing misfire. Particularly, since a decreasing speed of the in-cylinder temperature on the expansion stroke is higher as the compression ratio of the engine is higher, the ignition timing cannot be retarded very much.